Process for humidifying hardboard



Sept. 29, 1959 .1; J. ROBINSON, JR., EI' 2,906,034

I PROCESS FOR HUMIDIFYING HARDBOARD Filed March 15, 1957 PERCENT MOISTURE PEGAIN TIME INVENTOR JOSEPH J. ROBINSON JR.

CHARLES F. STORY 5 ATTORNEY United States Patent PROCESS FOR HUMIDIFYIN G HARDBOARD' Joseph J. Robinson, Jr., Dover, Ohio, and Charles F.

Story, Laurel, Miss., assiguors to Masonite Corporation, Laurel, Miss, a corporation of Delaware Application March 15, 1957, Serial No. 646,47 0

8 Claims. (CI. 34-26) The present invention relates to the finishing of lignocellulose hardboard products. More particularly, the invention relates to a novel method for effecting moisture regain in hardboard sheets having one surface sealed with a substantially moisture impervious coating material. The invention relates both to the novel method and to the treated products themselves.

Industrial fabricators of lignocellulose hardboard sheet articles have long been engaged in so-called finishing operations wherein the hardboard sheets are coated with such materials as, for example, resins, enamels, and the like coating agents. Usually, the coating composition has been applied to one surface, or face, of the hardboard sheet after which the coated article has been subjected to a-baking treatment in which the resin, enamel, or the like, is cured to its .iinal hard set. During the baking treatment, which is frequently 'elfected at temperatures of from 250 F. to about 300 F., the moisture content of the hardboard sheets is reduced substantially to zero. Subsequent to the baking treatment, the coated hardboard sheets have been subjected to a humidification treatment in order that the sheets may regain moisture up to whatever level is desired, usually of the order of'magnitude of about 2.5% to about It is necessary that the hardboard sheets contain approximately the equilibrium moisture content for normal atmospheric conditions prior to their sale and use in order to prevent excessive dimensional changes subsequent to their in stallation.

Obviously, the coated and baked hardboard sheets have. surfaces which are/unbalanced as regard their porosity and their ability to absorb moisture. Consequently, under many commonly employed humidifying conditions the treated sheets tend to warp inasmuch-as the uncoated surfaces, or faces, absorb moisture much more rapidly than do the coated surfaces. Conversely stated, the coated surfaces allow no appreciable penetration of moisture therethrough. Where the moisture regain is effected under conditions such that actual wetting of the sheet surfaces occurs, the individual fibers swell abnormally and unsightly surface rupture results. While a slight degree of rupture of the uncoated surfaces is sometimes permissible, it is never desirable. Rupture of the coated surface, however, is intolerable and hardboard sheets so disfigured are of no commercial value whatever.

Thus, for many years the art has faced the troublesome problem of moisture regain in coated hardboard sheet articles wherein the moisture must be regained through the one available permeable surface. The humidification must, of necessity, be carried out in such manner that the sheet surfaces remain smooth, unruptured, and otherwise unatfected. For the most part, the prior art humidification processes have entailed subjecting the coated articles to atmospheres of from medium to high relative humidity at low temperatures for long periods of time. For example, for a 3/ inch thick coated hardboardit has not been uncommon to resort to a 20 hour humidification cycle at about F. in order to obtain about 4-5% moisture in the article. Clearly, such a proce dure is highly uneconomical and undesirable in an art where it is not uncommon to process several hundred thousand square feet .of hardboard per day. Moreover, coated hardboard articles have invariably warped under such conditions.

The present invention has, as its principal object, the provision of a method for effecting moisture regain in coated hardboard articles quickly and without deleterious efiect to the treated articles.

Another object of the invention is the provision of a humidification method wherein the moisture regain of the hardboard quickly proceeds to equilibrium for any selected humidifying atmosphere within the critical range of the novel method.

A further object resides in the provision of a method wherein the rapid moisture regain is effected substantially completely through only one surface of the hardboard sheet but wherein the absorbed moisture is uniformly distributed throughout the sheet. Other objects of the invention will become apparent from the following dc.- tailed description thereof. The description will be made with particular reference to the drawing in which the single figure is a chart illustrating hardboard moisture regain cycles for various humidifying conditions.

Briefly, the invention stems from a two-fold discovery. First, it has been found that the rate of moisture regain invhardboard products increases in approximately direct proportion with increase in temperature and that the amount of moisture regained increases with increasing relative humidity at any given temperature. Second, under proper conditions which prevent appreciable and/ or prolonged condensation of moisture on the hardboard surfaces, relatively high temperatures as, for example, l90 F. may be employed for humidification. This discovery was entirely unexpected inasmuch as the prior art experience indicated that elevated temperatures and high humidity contributed very greatly to swelling and rupturing of the surface fibers in hardboard products. In carrying out the process of the invention at relatively high temperatures, however, the humidifying chamber employed must be constructed so as to permit adequate air flow, prevent condensation, and permit accurate humidity control.

A preferred type of humidification apparatus is dis.- closed and claimed in Copending application Serial No. 646,451 of Howard Max Wassem, filed on even date herewith. Essentially that apparatus comprises a double walled chamber equipped with gated openings for charging and discharging hardboard sheets into and out of the humidifier. The sheets are supported, in the humidifying chamber, by a wicket type conveyor wherein each wicket supports a single sheet in substantially vertical position. The walls of the inner chamber are constructed of material whose thermal conductivity is such'as to preclude condensation within the inner chamber under the particular humidifying conditions employed. That is to say, where a heat conductive structural material is used, the temperature of the inner chamber walls is maintained at a point relatively close to or above the wet bulb temperature of the humidifying chamber. Where a less conductive material is employed, the walls may be kept at a substantially lower temperature. The space between the inner and outer chambers is provided with heating equipment which is so regulated as to maintain the inner chamber walls at the desired temperature. The apparatus is also well adapted to provide desirable flow of the humidifying atmosphere. It will also be appreciated that the'humidifying apparatus is suited to either continuous operationor to batch-type humidification procedures since the conveyor mechanism can support a large 3 number of hardboard sheets in a relatively small space and the retention time of the sheets may be regulated as desired.

In carrying out the novel method of the .invention, therefore, the hardboard sheets are first coated on one surface with a selected coating composition, usually a film forming material which requires a baking treatment to cure it. The baked sheets normally contain substantially no moisture following the baking treatment. The sheets are then charged to the humidification chamber for moisture regain. As stated herein before, the dry bulb temperature employed, which represents the moisture driving force, may lie within the range of about 140 F. to about 190 F. Temperatures below about 140 F. require increasingly greater time cycles to effect the desired moisture regain, and temperatures appreciably above about 190 F. are not only diflicult to maintain, but may cause discoloration of the coating agent. Within this critical dry bulb temperature range, however, the wet bulb temperature is so selected as to effect the desired moisture regain under equilibrium conditions. In other words, the relative humidity of the chamber is so maintained that the desired moisture regain is obtained within a desired minimum time cycle and will not be appreciably exceeded if the time cycle is unduly extended for any reason. For practical purposes, the wet bulb temperature is maintained at a level within the range of from about 5 degrees to about 25 degrees below the dry bulb temperature. In commercial practice the wet bulb temperature necessary to obtain the desired relative humidity at any given dry bulb setting can easily be determined by referring to a standard psychrometric table. Furthermore, within the above temperature range, it is preferred that the dry bulb temperature be maintained from about 150 F. to about 175 F. inasmuch as this range offers the most desirable time cycles for moisture regain in coated hardboard sheets. The wet bulb setting will be determined by the percent moisture content desired in the panel.

The chart of Fig. 1 illustrates generally the moisture regain curves of coated hardboard sheets with the solid lines representing theoretical regain values and the dotted lines illustrating the actual deviation in regain from the theoretical. t will be seen that, at any given dry bulb temperature, the moisture regain is substantially a straight line function over a portion of the humidification period. However, as the moisture content of the sheets approaches equilibrium for the particular humidifying atmosphere employed, the rate of regan decreases resulting in a longer cycle than theoretical to reach equilibrium moisture regain. Within the critical limits hereinbefore set forth, this time lag may be tolerated. However, for the most commonly previously employed dry bulb temperature of 110 F., the time delay involved in attaining equilibrium was so great as to require the employment of humidifying cycles of such extended nature as to be economically impracticable. As shown in Fig. l, the actual rate of moisture regain at 110 F., shown by the dotted line, decreases to such an extent that the time involved is completely uneconomical. This is particularly true where about 5% of moisture is to be absorbed.

It will be further appreciated that, while the rate of moisture regain in coated hardboard articles increases approximately in direct proportion with increase in temperature, the regain rate is also atfected by the thickness and density of the articles to be conditioned. For example, a coated hardboard sheet A; inch thick and having a specific gravity of about 1.14 may absorb about 3% moisture in 2.5-3 hours at a temperature of about 170 F. and relative humidity of about 68%. However, a similar sheet X inch thick and having a specific gravity somewhat lower than 1.1 will absorb a like amount of moisture in a slightly shorter period of time. At dry bulb temperatures of about 190 F, and wet bulb tem- 4. peratures of about 180 F., the same hardboard articles will absorb a like amount of moisture in slightly more than 30 minutes. Obviously, the lower temperature of 140 F. will correspondingly extend the humidification cycle.

An outstanding feature of the novel method concerns the temperature of coated hardboard sheets as they are charged to the humidification chamber. It was previously believed by the art that the sheets to be conditioned must be heated to such a degree as to prevent any condensation of moisture on the sheet surfaces as they entered the chamber. This belief was predicated on the prior art experience with uncoated boards which present two pervious surfaces. The larger fibers which are always present to some degree in hardboard surfaces were known to absorb the condensed moisture which was present on the surfaces of cold-charged sheets so rapidly as to swell these larger fibers. Once swollen, the enlarged fibers never reverted to their original size. Thus, the sheet surfaces were rough and unsightly subsequent to humidification.

It was discovered, quite unexpectedly and contrary to prediction, that coated hardboard sheets at room temperature may be charged to the humidifier where the humidifying atmosphere is within the previously stated critical limits. There is an initial substantially instantaneous condensation of moisture on the surfaces of the sheets which may even be visually observed. However, the duration of this moisture film is shortlived and there is no enlargement at all of the surface fibers on the coated surfaces and only a very slight enlargement of the large surface fibers on the uncoated surfaces. This discovery is of great importance inasmuch as it is frequently desirable to charge to the humidifier hardboard sheets which have returned to room temperature subsequent to their being baked. The elimination of an additional heating treatment results in a decided economic advantage. It is not only surprising that moisture may be driven uniformly into dense, coated hardboard sheets from one surface only, but it is even more startling that cool sheets may be charged to an atmosphere of both high temperature and humidity without damage to the coated sheets.

Subsequent to the humidifying cycle, the treated hard board sheets are discharged from the humidifying chamher. The sheets are stacked, preferably in facc-to-face and back-to-back relationship to avoid marring the coated face surfaces. The stacking procedure is essential although obviously the sheets may be stacked as desired where there is no danger of harm to the coated surfaces. Inasmuch as the conditioned sheets are quite warm and may even be very hot upon discharge to an area of room temperature, their regained moisture escapes readily therefrom during cooling of the sheets. Stacking of the sheets effectively prevents any appreciable moisture escape, thereby also minimizing any tendency of the sheets to warp.

It will be seen from the foregoing description that the present invention presents to the art a novel humidification method particularly adapted to moisture regain in coated hardboard articles. Where it has previously been necessary to humidify such articles at low temperature and for periods of time which have often exceeded 20 hours, the method of the present invention enables the art to accomplish a much more uniform, desirable moisture regain in from about '50 minutes to about 6 hours maximum. It may be stated categorically that at dry bulb temperatures approaching 190 F. and wet bulb temperatures which provide -85% relative humidity, about 5% moisture may be forced into dense coated hardboard sheets in a period of about 2 to 2.5 hours. At the lower dry bulb temperature of F., about 6 hours are required to effect the same moisture regain. These time cycles are applicable to coated hardboard sheets of thicknesses of about A; to $4 inch and specific gravities of 1.1 or higher. The absorbed moisture is distributed uniformly throughout the sheets, as evidenced by s b tantial freedom of the sheets from any tendency to warp. The humidification method is adapted to both continuous and batch-type operation and presents a decided economic improvement over prior art procedures.

We claim:

1. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets to a closed chamber in spacedapart position, subjecting said sheets to a humid atmosphere at about 140 F.-l90 F. dry bulb temperature and a Wet bulb temperature within the range of about 5 to about 25 below said dry bulb temperature for a period of time sufiicient for the said hardboard sheets to absorb about 2%5% by weight of moisture, said heated atmosphere having a relative humidity such that the said range of absorbed moisture represents substantially equilibrium moisture uptake within said dry bulb temperature range, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal .surface-to-surface relationship, and cooling said sheets to room temperature in stacked condition.

2. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets at room temperature to a closed chamber in spaced-apart position, subjecting said sheets to a humid atmosphere at about 140 F.190 F. dry bulb temperature and a Wet bulb temperature Within the range of about 5 to about 25 below said dry bulb temperature for a period of time sufiicient for the said hardboard sheets to absorb about 2%5% by weight of moisture, said heated atmosphere having a relative humidity such that the said range of absorbed moisture represents substantially equilibrium moisture uptake within said dry bulb temperature range, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal surface-to-surface relationship, and cooling said sheets to room temperature in stacked condition.

3. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets to a closed chamber in spacedapart position, subjecting said sheets to a humid atmosphere at about 150 F.l75 F. dry bulb temperature and a Wet bulb temperature within the range of about 5 to about 25 below said dry bulb temperature for a period of time sufficient for the said hardboard sheets to absorb about 2%5% by weight of moisture, said heated atmosphere having a relative humidity such that the said range of absorbed moisture represents equilibrium moisture uptake within said dry bulb temperature range, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal surface-to-surface relationship, and cooling said sheets to room temperature in stacked condition.

4. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets at room temperature to a closed chamber in spaced-apart position, subjecting said sheets to a humid atmosphere at about 150 F.l75 F. dry bulb temperature and a wet bulb temperature within the range of about 5 to about 25 below said dry bulb temperature for a period of time sufficient for the said hardboard sheets to absorb about 2%5% by weight of moisture, said heated atmosphere having a relative humidity such that the said range of absorbed moisture represents equilibrium moisture uptake within said dry bulb temperature range, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal surface-to-surface relationship, and cooling said sheets to room temperature in stacked condition.

5. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets to a closed chamber in spacedapart position, subjecting said sheets to a humid atmosphere at about F.190 F. dry bulb temperature and a wet bulb temperature within the range of about 5 to about 25 below said dry bulb temperature for a period of time within the range of about 1.5 to about 6 hours, said atmosphere having a relative humidity such that the hardboard sheets absorb an equilibrium amount of moisture within the range of about 2% to about 5% by weight, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal surface-to-surface relationship, and cooling said sheets to room temperature in stacked condition.

6. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets at room temperature to a closed chamber in spaced-apart position, subjecting said sheets to a humid atmosphere at about 140 F.190 F. dry bulb temperature and a wet bulb temperature within the range of about 5 to about 25 below said dry bulb temperature for a period of time within the range of about 1.5 to about 6 hours, said atmosphere having a relative humidity such that the hardboard sheets absorb an equilibrium amount of moisture within the range of about 2% to about 5% by weight, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal surface-to-surface relationship, and cooling said sheets to room temperature in stacked condition.

7. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets to a closed chamber in spaced apart position, subjecting said sheets to a humid atmosphere at about F. F. dry bulb temperature and a wet bulb temperature within the range of about 5 to about 25 below said dry bulb temperature for a period of time within the range of about 1.5 to about 6 hours, said atmosphere having a relative humidity such that the hardboard sheets absorb an equilibrium amount of moisture Within the range of about 2% to about 5% by weight, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal surfaceto-surface relationship, and cooling said sheets to room temperature in stacked condition.

8. A method of humidifying dense hardboard sheets having one surface thereof sealed with a substantially impervious coating material which comprises charging said hardboard sheets at room temperature to a closed chamber in spaced-apart position, subjecting said sheets to a humid atmosphere at about 150 F.l75 F. dry bulb temperature and a wet bulb temperature within the range of about 5 to about 25 below said dry bulb temperature for a period of time within the range of about 1.5 to about 6 hours, said atmosphere having a relative humidity such that the hardboard sheets absorb an equilibrium amount of moisture within the range of about 2% to about 5% by weight, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal surface-to-surface relationship, and cooling said sheets to room temperature in stacked condition.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,071 Nevin Apr. 14, 1942 1,160,724 Landin Nov. 16, 1915 2,284,838 Oholm June 2, 1942 2,728,444 Richardson et a1 Dec. 27, 1955 

